1. Field of the Invention
The present invention relates to an image forming apparatus employing an electrophotographic process, such as photocopying machines, laser beam printers, facsimile apparatuses.
2. Description of the Related Art
Generally, a typical image forming apparatus employing an electrophotographic process includes a rotatably-supported photoconductive drum 1 as shown in FIG. 2. The photoconductive drum 1 is driven by drive means (not shown) to rotate in a direction S1. Around the periphery of the photoconductive drum 1 are disposed a charging device 2, an exposing device 3, a developing device 4, a transferring device 5, and a cleaning device 6 in sequential order from upstream of the rotational direction S1. A surface of the photoconductive drum 1 charged in a uniform manner by the charging device 2 is exposed to laser light by the exposing device 3, thereby to form an electrostatic latent image. The electrostatic latent image is developed with toner by the developing device 4 to become a toner image. Then, the toner image is transferred onto a transfer material by the transferring device 5. After the transfer process, the cleaning device 6 is disposed to remove residual toner present on the photoconductive drum 1.
As shown in FIG. 2, the color image forming apparatus comprises serially-arranged four units each comprising the photoconductive drum 1, the charging device 2, the exposing device 3, the developing device 4, the transferring device 5, and the cleaning device 6, so that images of Y (yellow), M (magenta), C (cyan), and BK (black) are individually formed. The developing devices 4 of these four units have developer tanks for holding therein toner of Y, toner of M, toner of C, and toner of BK, respectively.
A transfer belt 7 forming the transferring device 5 of each unit is common to the four units. The transfer belt 7 composed of denatured polyimide or the like material is constructed such that it is stretched between a pulley 17 on the drive side and a pulley 18 on the driven side in order to be able to travel in a direction S2, and transfer material such as a sheet of paper is electrostatically adsorbed to the transfer belt 7 and held there. Inside the transfer belt 7 are disposed transfer chargers 8 for transferring a toner image formed on the photoconductive drum 1 to the transfer material, to face their corresponding photoconductive drums 1, respectively. Moreover, a cleaning member for cleaning toner accumulated on the transfer belt 7 and an electric-charge eliminating member for discharging electric charge accumulated on the transfer belt 7, which are not shown in the figure, are provided inside or outside the transfer belt 7. Toner images of Y, M, C and BK transferred onto the transfer material are fixed by a fixing device 9, whereby a color image is formed.
The density characteristics of the formed image will vary with, for example, surrounding temperature and humidity and time. In other words, under the influence of variation factors of each of image formation processes such as charging, exposing, developing, transferring and fixing, image density characteristics change. To cope with such variations and obtain a constant image density, a correction to the image density is made. The amount of correction in the image density correction is determined in such a manner that a toner pattern 15 composed of a plurality of toner patches is formed on the transfer belt 7, the densities of the toner patches are detected by a density detecting sensor 10, and detection values are compared with a predetermined reference value, whereby amounts of correction to be made are determined. Image recording apparatuses of the type in which such an image density correction is made are disclosed in, for example, Japanese Unexamined Patent Publications JP-A 8-289148 (1996), JP-A 8-289153 (1996), and JP-A 8-340448 (1996).
The image output starting point obtained by increasing the exposure width of the exposing device 3 from its minimum value, i.e., the tone rise point, varies with the surrounding environment and time, as mentioned above. Although it is essential to stably obtain an accurate tone rise point when performing a tone representation (particularly a light color tone representation), the amount of toner adhesion at a tone rise point is extremely small and the density is, therefore, considerably low. Consequently, there are some image densities in a low-density region, undetectable by the density detecting sensor 10 due to the level of sensitivity thereof. Therefore, in the case where an exposure width which was detected by the density detecting sensor is employed as the tone rise point, there occurs a disagreement in exposure width with an actual density. Moreover, there are some image densities in a high-density region that cannot be detected with high accuracy by the density detecting sensor 10 due to the level of sensitivity.